1. Field of the Invention
The present invention relates to a lithographic apparatus comprising a radiation source having a debris-mitigation system and method for mitigating debris particles in a lithographic apparatus.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
During production of EUV radiation, as well-known in the art, debris particles are formed as a by-product. The production of EUV radiation occurs by heating an object or a gas by means of a laser source of high energy density. The object or gas will produce secondary emission, EUV radiation, as well as undesired particles. These debris particles may have a detrimental effect on the performance of the lithographic apparatus as these debris particles may collect on, for example, optical elements. The debris particles present on these optical elements may hinder reflection or transmission of the radiation beam. Debris particles with relatively high kinetic energy may even cause serious damage to the apparatus by, for example, etching of the optical elements which are very expensive and very crucial parts of the apparatus. Hence, the need for mitigating debris particles.
The term “mitigating” is meant to encompass any manipulation of the debris particles which may lead to at least a reduction of the detrimental effect on the performance of the lithographic apparatus, a reduction of the damage to the lithographic apparatus, or both. The term mitigating also encompasses thermolization of particles. Thermolization is understood to be a form of manipulating the behavior of a gas or plasma such that a peak in the energy- or velocity distribution is evened out and all the particles will behave in correspondence with a stochastically derivable energy- or velocity distribution function.
A lithographic apparatus with a debris-mitigation system is described in WO99/42904. The debris-mitigation system known from this prior art comprises a filter, in particular a so-called foil trap, which may comprise a plurality of foils or plates which capture the debris particles. The plates or foils are as much as possible oriented parallel to a radiation path so that radiation will pass the filter.
Although the foil trap is useful, a lithographic apparatus with an additional or alternative debris-mitigation systems is required to provide for improved mitigation of debris particles and improved flexibility in application of a debris-mitigation system and/or allow for more freedom in the design of a lithographic apparatus.